In many medical procedures, such as the treatment of benign or malignant tumors, it is important to be able to ablate the undesirable tissue in a controlled and focused way without affecting the surrounding desirable tissue. Over the years, a large number of minimally invasive methods have been developed to selectively destroy specific areas of undesirable tissues as an alternative to resection surgery. There are a variety of techniques with specific advantages and disadvantages, which are indicated and contraindicated for various applications. For example, cryosurgery is a low temperature minimally invasive technique in which tissue is frozen on contact with a cryogen cooled probe inserted in the undesirable tissue (Rubinsky, B., ed. Cryosurgery. Annu. Rev. Biomed. Eng. Vol. 2. 2000. 157-187.). The area affected by low temperature therapies, such as cryosurgery, can be easily controlled through imaging. However, the probes are large and difficult to use. Non-selective chemical ablation is a technique in which chemical agents such as ethanol are injected in the undesirable tissue to cause ablation (Shiina, S., et al., Percutaneous ethanol injection therapy for hepatocellular carcinoma: results in 146 patients. AJR, 1993. 160: p. 1023-8). Non-selective chemical therapy is easy to apply. However, the affected area cannot be controlled because of the local blood flow and transport of the chemical species. Elevated temperatures are also used to ablate tissue. Focused ultrasound is a high temperature non-invasive technique in which the tissue is heated to coagulation using high-intensity ultrasound beams focused on the undesirable tissue (Lynn, J. G., et al., A new method for the generation of use of focused ultrasound in experimental biology. J. Gen Physiol., 1942. 26: p. 179-93; Foster, R. S., et al., High-intensity focused ultrasound in the treatment of prostatic disease. Eur. Urol., 1993. 23: p. 44-7). Electrical currents are also commonly used to heat tissue. Radiofrequency ablation (RF) is a high temperature minimally invasive technique in which an active electrode is introduced in the undesirable tissue and a high frequency alternating current of up to 500 kHz is used to heat the tissue to coagulation (Organ, L. W., Electrophysiological principles of radiofrequency lesion making. Appl. Neurophysiol., 1976. 39: p. 69-76). In addition to RF heating traditional Joule heating methods with electrodes inserted in tissue and dc or ac currents are also common, (Erez, A., Shitzer, A. (Controlled destruction and temperature distribution in biological tissue subjected to monoactive electrocoagulation) J. Biomech. Eng. 1980:102(1):42-9). Interstitial laser coagulation is a high temperature thermal technique in which tumors are slowly heated to temperatures exceeding the threshold of protein denaturation using low power lasers delivered to the tumors by optical fibers (Bown, S. G., Phototherapy of tumors. World. J. Surgery, 1983. 7: p. 700-9). High temperature thermal therapies have the advantage of ease of application. The disadvantage is the extent of the treated area is difficult to control because blood circulation has a strong local effect on the temperature field that develops in the tissue. The armamentarium of surgery is enhanced by the availability of the large number of minimally invasive surgical techniques in existence, each with their own advantages and disadvantages and particular applications. This document discloses another minimally invasive surgical technique for tissue ablation, irreversible electroporation. We will describe the technique, evaluate its feasibility through mathematical modeling and demonstrate the feasibility with in vivo experimental studies.
Electroporation is defined as the phenomenon that makes cell membranes permeable by exposing them to certain electric pulses (Weaver, J. C. and Y. A. Chizmadzhev, Theory of electroporation: a review. Bioelectrochem. Bioenerg., 1996. 41: p. 135-60). Electroporation pulses are defined as those electrical pulses that through a specific combination of amplitude, shape, time length and number of repeats produce no other substantial effect on biological cells than the permeabilization of the cell membrane. The range of electrical parameters that produce electroporation is bounded by: a) parameters that have no substantial effect on the cell and the cell membrane, b) parameters that cause substantial thermal effects (Joule heating) and c) parameters that affect the interior of the cell, e.g. the nucleus, without affecting the cell membrane. Joule heating, the thermal effect that electrical currents produce when applied to biological materials is known for centuries. It was noted in the previous paragraph that electrical thermal effects which elevate temperatures to values that damage cells are commonly used to ablate undesirable tissues. The pulse parameters that produce thermal effects are longer and/or have higher amplitudes than the electroporation pulses whose only substantial effect is to permeabilize the cell membrane.
There are a variety of methods to electrically produce thermal effects that ablate tissue. These include RF, electrode heating, and induction heating. Electrical pulses that produce thermal effects are distinctly different from the pulses which produce electroporation. The distinction can be recognizing through their effect on cells and their utility. The effect of the thermal electrical pulses is primarily on the temperature of the biological material and their utility is in raising the temperature to induce tissue ablation through thermal effects.
The effect of the electroporation parameters is primarily on the cell membrane and their utility is in permeabilizing the cell membrane for various applications. Electrical parameters that only affect the interior of the cell, without affecting the cell membrane were also identified recently. They are normally referred to as “nanosecond pulses”. It has been shown that high amplitude, and short (substantially shorter than electroporation pulses—nanoseconds versus millisecond) length pulses can affect the interior of the cell and in particular the nucleus without affecting the membrane. Studies on nanosecond pulses show that they are “distinctly different than electroporation pulses” (Beebe S J. Fox P M. Rec L J: Somers K. Stark R H. Schoenbach K H. Nanosecond pulsed electric field (nsPEF) effects on cells and tissues: apoptosis induction and tumor growth inhibition. PPPS-2001 Pulsed Power Plasma Science 2001. 28th IEEE International Conference on Plasma Science and 13th IEEE International Pulsed Power Conference. Digest of Technical Papers (Cat. No. 01CH37251). IEEE. Part vol. 1, 2001, pp. 211-15 vol. 1. Piscataway, N.J., USA. Several applications have been identified for nano-second pulses. One of them is for tissue ablation through an effect on the nucleus (Schoenbach, K. H., Beebe, S. J., Buescher, K. S. Method and apparatus for intracellular electro-manipulation U.S. Patent Application Pub No. US 2002/0010491 A1, Jan. 24, 2002). Another is to regulate genes in the cell interior, (Gunderson, M. A. et al. Method for intracellular modification within living cells using pulsed electrical fields—regulate gene transcription and entering intracellular US Patent application 2003/0170898 A1, Sep. 11, 2003). Electrical pulses that produce intracellular effects are distinctly different from the pulses which produce electroporation. The distinction can be recognizing through their effect on cells and their utility. The effect of the intracellular electrical pulses is primarily on the intracellular contents of the cell and their utility is in manipulating the intracellular contents for various uses—including ablation. The effect of the electroporation parameters is primarily on the cell membrane and their utility is in permeabilizing the cell membrane for various applications, which will be discussed in greater detail later.
Electroporation is known for over half a century. It was found that as a function of the electrical parameters, electroporation pulses can have two different effects on the permeability of the cell membrane. The permeabilization of the membrane can be reversible or irreversible as a function of the electrical parameters used. In reversible electroporation the cell membrane reseals a certain time after the pulses cease and the cell survives. In irreversible electroporation the cell membrane does not reseal and the cell lyses. A schematic diagram showing the effect of electrical parameters on the cell membrane permeabilization (electroporation) and the separation between: no effect, reversible electroporation and irreversible electroporation is shown in FIG. 1 (Dev, S. B., Rabussay, D. P., Widera, G., Hofmann, G. A., Medical applications of electroporation, IEEE Transactions of Plasma Science, Vol 28 No 1, February 2000, pp 206-223) Dielectric breakdown of the cell membrane due to an induced electric field, irreversible electroporation, was first observed in the early 1970s (Neumann, E. and K. Rosenheck, Permeability changes induced by electric impulses in vesicular membranes. J. Membrane Biol., 1972. 10: p. 279-290; Crowley, J. M., Electrical breakdown of biomolecular lipid membranes as an electromechanical instability. Biophysical Journal, 1973. 13: p. 711-724; Zimmermann, U., J. Vienken, and G. Pilwat, Dielectric breakdown of cell membranes, Biophysical Journal, 1974. 14(11): p. 881-899). The ability of the membrane to reseal, reversible electroporation, was discovered separately during the late 1970s (Kinosita Jr, K. and T. Y. Tsong, Hemolysis of human erythrocytes by a transient electric field. Proc. Natl. Acad. Sci. USA, 1977. 74(5): p. 1923-1927; Baker, P. F. and D. E. Knight, Calcium-dependent exocytosis in bovine adrenal medullary cells with leaky plasma membranes. Nature, 1978. 276: p. 620-622; Gauger, B. and F. W. Bentrup, A Study of Dielectric Membrane Breakdown in the Fucus Egg, J. Membrane Biol., 1979. 48(3): p. 249-264).
The mechanism of electroporation is not yet fully understood. It is thought that the electrical field changes the electrochemical potential around a cell membrane and induces instabilities in the polarized cell membrane lipid bilayer. The unstable membrane then alters its shape forming aqueous pathways that possibly are nano-scale pores through the membrane, hence the term “electroporation” (Chang, D. C., et al., Guide to Electroporation and Electrofusion. 1992, San Diego, Calif.: Academic Press, Inc.). Mass transfer can now occur through these channels under electrochemical control. Whatever the mechanism through which the cell membrane becomes permeabilized, electroporation has become an important method for enhanced mass transfer across the cell membrane.
The first important application of the cell membrane permeabilizing properties of electroporation is due to Neumann (Neumann, E., et al., Gene transfer into mouse lyoma cells by electroporation in high electric fields. J. EMBO, 1982. 1: p. 841-5). He has shown that by applying reversible electroporation to cells it is possible to sufficiently permeabilize the cell membrane so that genes, which are macromolecules that normally are too large to enter cells, can after electroporation enter the cell. Using reversible electroporation electrical parameters is crucial to the success of the procedure, since the goal of the procedure is to have a viable cell that incorporates the gene.
Following this discovery electroporation became commonly used to reversible permeabilize the cell membrane for various applications in medicine and biotechnology to introduce into cells or to extract from cells chemical species that normally do not pass, or have difficulty passing across the cell membrane, from small molecules such as fluorescent dyes, drugs and radioactive tracers to high molecular weight molecules such as antibodies, enzymes, nucleic acids, HMW dextrans and DNA. It is important to emphasize that in all these applications electroporation needs to be reversible since the outcome of the mass transport requires for the cells to be alive after the electroporation.
Following work on cells outside the body, reversible electroporation began to be used for permeabilization of cells in tissue. Heller, R., R. Gilbert, and M. J. Jaroszeski, Clinical applications of electrochemotherapy. Advanced drug delivery reviews, 1999. 35: p. 119-129. Tissue electroporation is now becoming an increasingly popular minimally invasive surgical technique for introducing small drugs and macromolecules into cells in specific areas of the body. This technique is accomplished by injecting drugs or macromolecules into the affected area and placing electrodes into or around the targeted tissue to generate reversible permeabilizing electric field in the tissue, thereby introducing the drugs or macromolecules into the cells of the affected area (Mir, L. M., Therapeutic perspectives of in vivo cell electropermeabilization. Bioelectrochemistry, 2001. 53: p. 1-10).
The use of electroporation to ablate undesirable tissue was introduced by Okino and Mohri in 1987 and Mir et al. in 1991. They have recognized that there are drugs for treatment of cancer, such as bleomycin and cys-platinum, which are very effective in ablation of cancer cells but have difficulties penetrating the cell membrane. Furthermore, some of these drugs, such as bleomycin, have the ability to selectively affect cancerous cells which reproduce without affecting normal cells that do not reproduce. Okino and Mori and Mir et al. separately discovered that combining the electric pulses with an impermeant anticancer drug greatly enhanced the effectiveness of the treatment with that drug (Okino, M. and H. Mohri, Effects of a high-voltage electrical impulse and an anticancer drug on in vivo growing tumors. Japanese Journal of Cancer Research, 1987. 78(12): p. 1319-21; Mir, L. M., et al., Electrochemotherapy potentiation of antitumour effect of bleomycin by local electric pulses. European Journal of Cancer, 1991. 27: p. 68-72). Mir et al. soon followed with clinical trials that have shown promising results and coined the treatment electrochemotherapy (Mir, L. M., et al., Electrochemotherapy, a novel antitumor treatment: first clinical trial. C. R. Acad. Sci., 1991. Ser. III 313(613-8)).
Currently, the primary therapeutic in vivo applications of electroporation are antitumor electrochemotherapy (ECT), which combines a cytotoxic nonpermeant drug with permeabilizing electric pulses and electrogenetherapy (EGT) as a form of non-viral gene therapy, and transdermal drug delivery (Mir, L. M., Therapeutic perspectives of in vivo cell electropermeabilization. Bioelectrochemistry, 2001. 53: p. 1-10). The studies on electrochemotherapy and electrogenetherapy have been recently summarized in several publications (Jaroszeski, M. J., et al., In vivo gene delivery by electroporation. Advanced applications of electrochemistry, 1999. 35: p. 131-137; Heller, R., R. Gilbert, and M. J. Jaroszeski, Clinical applications of electrochemotherapy. Advanced drug delivery reviews, 1999. 35: p. 119-129; Mir, L. M., Therapeutic perspectives of in vivo cell electropermeabilization. Bioelectrochemistry, 2001. 53: p. 1-10; Davalos, R. V., Real Time Imaging for Molecular Medicine through electrical Impedance Tomography of Electroporation, in Mechanical Engineering. 2002, University of California at Berkeley: Berkeley. p. 237). A recent article summarized the results from clinical trials performed in five cancer research centers. Basal cell carcinoma (32), malignant melanoma (142), adenocarcinoma (30) and head and neck squamous cell carcinoma (87) were treated for a total of 291 tumors (Mir, L. M., et al., Effective treatment of cutaneous and subcutaneous malignant tumours by electrochemotherapy. British Journal of Cancer, 1998. 77(12): p. 2336-2342).
Electrochemotherapy is a promising minimally invasive surgical technique to locally ablate tissue and treat tumors regardless of their histological type with minimal adverse side effects and a high response rate (Dev, S. B., et al., Medical Applications of Electroporation. IEEE Transactions on Plasma Science, 2000. 28(1): p. 206-223; Heller, R., R. Gilbert, and M. J. Jaroszeski, Clinical applications of electrochemotherapy. Advanced drug delivery reviews, 1999. 35: p. 119-129). Electrochemotherapy, which is performed through the insertion of electrodes into the undesirable tissue, the injection of cytotoxic drugs in the tissue and the application of reversible electroporation parameters, benefits from the ease of application of both high temperature treatment therapies and non-selective chemical therapies and results in outcomes comparable of both high temperature therapies and non-selective chemical therapies.
In addition, because the cell membrane permeabilization electrical field is not affected by the local blood flow, the control over the extent of the affected tissue by this mode of ablation does not depend on the blood flow as in thermal and non-selective chemical therapies. In designing electroporation protocols for ablation of tissue with drugs that are incorporated in the cell and function in the living cells it was important to employ reversible electroporation; because the drugs can only function in a living cell. Therefore, in designing protocols for electrochemotherapy the emphasis was on avoiding irreversible electroporation. The focus of the entire field of electroporation for ablation of tissue was on using reversible pulses, while avoiding irreversible electroporation pulses, that can cause the incorporation of selective drugs in undesirable tissue to selectively destroy malignant cells. Electrochemotherapy which employs reversible electroporation in combination with drugs, is beneficial due to its selectivity however, a disadvantage is that by its nature, it requires the combination of chemical agents with an electrical field and it depends on the successful incorporation of the chemical agent inside the cell.
An important concern in the studies of electrochemotherapy and electrogenetherapy in living tissue is the effect of electroporation on blood flow. Martin et al., have found that when reversible electroporation is used for introducing genes into cells on the blood vessel wall the blood vessels remain intact and their response to stimuli where indistinguishable from those of control vessels (Martin, J. B., Young, J. L., Benoit, J. N., Dean, D. A., Gene transfer to intact Mesenteric arteries by electroporation, Journal of vascular research, 2000, Vol 37:372-380). Ivanusa et al have found using MRI that with certain electroporation pulses, which appear to be in the irreversible electroporation range, that the electroporation transiently but significantly reduced tumor blood flow (Ivanusa, T, Berays, K., Cemazar, M., Jevtic, V, Demsar, F., Sersa, G. MRI macromolecular contrast agents as indicators of changed tumor blood flow, Radiol. Oncol. 2001; 35(2): 139-47). These findings are very different from those described here.
Sersa et al performed studies whose goal was to determine the effect of electrochemotherapy, reversible electroporation with bleomycin or cisplatin, on tumor blood flow (Sersa, G., Sentjurc, M., Ivanusa, T, Berays, K., Kotnik, V, Coer, A., Swartz, H. M., Cemazar, M. Reduced blood flow and axygenation in SA-1 tumours after electrochemotherapy with cisplatin, Br. J. Cancer, 2002: 87(9):1047-54) (Sersa, G., Cemazar, M., Miklavcic, D. Tumor blood flow modifying effects of electrochemotherapy: a potential targeted mechanism radiol. Oncol 2003: 37(1): 43-8). In the first of the papers they report reduced blood flow that persisted for several days when using reversible electroporation with cisplatin. In the second paper they report complete shut down of blood flow after 24 hours when using reversible electroporation with bleomycin and 50% reduction in blood flow when using reversible electroporation with cisplatin.